K&R style function definition compile using g++

Question

When compiling a c file that uses old style function definition like

int foo(a)
   int a;
{
  ...
}

g++ will give and error: ‘a’ was not declared in this scope. gcc can parse this. Is there a way to let g++ recognize this?

This comes up as an issue to me because I'm compiling a mix of c and c++ files. A related question is what's the standard practice of building this type of mixed source? Running g++ on all files or only the cc files? The former is convenient but keeps getting me some trouble because of the inconsistencies between c and c++ specification(for example, char[4]="four";)

Answer 1

Is there a way to let g++ recognize this?

This syntax is not supported in C++.

Running g++ on all files or only the cc files?

See e.g. Compiling C++ programs from the GCC docs:

C++ source files conventionally use one of the suffixes .C', .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats .c, .h and .i files as C++ source files instead of C source files unless -x is used, and automatically specifies linking against the C++ library. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations.

So two possibilities:

1. Run gcc on C files, and g++ on C++ files.
2. Run gcc on all files.

In both cases you will need to link with g++ (or gcc -lstdc++).

Answer 2

Oli is correct: C++ doesn't support old-style function definitions.

Compile C with a C compiler (such as gcc).

Compile C++ with a C++ compiler (such as g++).

They're two different (though closely related) languages. You can use C++'s extern "C" feature to invoke C code from C++ and vice versa; see section 32 of the C++ FAQ Lite for more information.

Answer 3

If you are going to compile both C and C++, you are better off compiling always with gcc (it will choose the language based on the file extension) than g++ (will always compile as C++). You will need to change your linker options to include C++ standard library (and -lm if you use it) as those are automatically added by g++ but not gcc.

Alternatively, a better option is to call the g++ for C++ and gcc for C files. That should not be too hard to manage by configuring the build system.